/*
 * Copyright (c) 1997, 2003, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang.ref;


/**
 * Phantom reference objects, which are enqueued after the collector
 * determines that their referents may otherwise be reclaimed.  Phantom
 * references are most often used for scheduling pre-mortem cleanup actions in
 * a more flexible way than is possible with the Java finalization mechanism.
 *
 * <p> If the garbage collector determines at a certain point in time that the
 * referent of a phantom reference is <a
 * href="package-summary.html#reachability">phantom reachable</a>, then at that
 * time or at some later time it will enqueue the reference.
 *
 * <p> In order to ensure that a reclaimable object remains so, the referent of
 * a phantom reference may not be retrieved: The <code>get</code> method of a
 * phantom reference always returns <code>null</code>.
 *
 * <p> Unlike soft and weak references, phantom references are not
 * automatically cleared by the garbage collector as they are enqueued.  An
 * object that is reachable via phantom references will remain so until all
 * such references are cleared or themselves become unreachable.
 *
 * @author Mark Reinhold
 * @since 1.2
 */

public class PhantomReference<T> extends Reference<T> {

  /**
   * Returns this reference object's referent.  Because the referent of a
   * phantom reference is always inaccessible, this method always returns
   * <code>null</code>.
   *
   * @return <code>null</code>
   */
  public T get() {
    return null;
  }

  /**
   * Creates a new phantom reference that refers to the given object and
   * is registered with the given queue.
   *
   * <p> It is possible to create a phantom reference with a <tt>null</tt>
   * queue, but such a reference is completely useless: Its <tt>get</tt>
   * method will always return null and, since it does not have a queue, it
   * will never be enqueued.
   *
   * @param referent the object the new phantom reference will refer to
   * @param q the queue with which the reference is to be registered, or <tt>null</tt> if
   * registration is not required
   */
  public PhantomReference(T referent, ReferenceQueue<? super T> q) {
    super(referent, q);
  }

}
